I was all ready to “um, actually” this, but, um, actually there’s about 3-4 grams of iron in a person, which x400 is 1.2-1.6kg, which is a smallish but not unreasonable sword. So. Math checks out.

How would you extract the iron, though? The more practical solution would be to kill a mere hundred men, then mix 1 part blood with 3 parts standard molten iron, imo. Cheaper and faster, while still retaining the edge that only evil magic can give you.

Or, you could just make the sword of iron, and then use the blood to temper the blade.

1.2 to 1.6 kilograms is a perfectly reasonable large sword. Your average longsword was 1.1–1.8 kg and I don’t even remember if that’s including the weight of the hilt, guard, and pommel or just the blade. Your more classic “knight sword” was a mere 1.1 kilograms on average; the blood of 400 men is more than enough.

This is using the comparatively crappy metallurgy of medieval Europe and their meh iron swords. Move east to, say, contemporary Iran and make a scimitar using high carbon steel (~2%) for a .75 kilogram blade and you only need the blood of about 225 men.

So putting my thoughts in on this… because how could I not.

So you’ve exsanguinated your 400 guys to get the iron for your sword. Cool. But now you have 400 bodies lying around.

Why not put those to good use and cremate them. Use the carbon from those 400 bodies (you won’t need all of them) and now you can make a nice mid-high carbon steel sword.

Now you have a sword forged with the blood of your enemies AND strengthened with their bones.

“high fantasy math” - the tag I should have expected to write some day.